The murky details of IPv6 implementations never crop up till you start deploying it (or, as Randy Bush recently wrote: “it is cheering to see that the ipv6 ivory tower still stands despite years of attack by reality”).

Here’s another one: in theory the prefixes delegated through DHCPv6 should be static and permanently assigned to the customers for long periods of time.

Quoting from RFC 3633: Even though this mechanism makes automatic renumbering easier, it is expected that
prefixes have a long lifespan. During renumbering it is expected that the old and the new prefix co-exist for some time. It's almost impossible to implement coexistence of old and new prefixes with today's BRAS software; the only reliable approach is to make delegated prefixes as stable as possible.

In practice, you never know how many of your customers will actually ask for the delegated prefix, so it might be simpler to use a local BRAS pool to serve the customers that ask for a delegated prefix out of the blue, and static RADIUS-based prefix delegation for people who actually ask for a static prefix.

If I understand it correctly the problem is not the BRAS pool its the CPE.After PD subnet change the CPE should send RA on it LAN with lifetime 0 for the old prefix and send RA with non-zero lifetime for the new prefix.See L-13 in http://tools.ietf.org/html/draft-ietf-v6ops-6204bis-08I hope they will do it correctly or we will have a lot of problems.

I was unaware that the delegated prefix was assumed to be static in theory...

My own provider also gives me the same delegated prefix (through AAA) but without any guarantee as my BRAS could change if they need to redesign their network.

And for having worked on DHCP-PD on low cost CPE code, this is indeed tricky because all low cost CPE (or at least most) rely on Linux and there is little interconnection between the several buggy DHCPv6 clients and RADVD or quagga or ...

The author

Ivan Pepelnjak (CCIE#1354 Emeritus), Independent Network Architect at ipSpace.net,
has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced internetworking technologies since 1990.